MIMO (Multiple-Input Multiple-Output) technology means a method for improving efficiency in data transmission and reception by using multiple transmitting antennas and multiple receiving antennas instead of a single transmitting antenna and a single receiving antenna. Namely, the MIMO technology is to increase capacity or improve throughput by using multiple antennas in a transmitting end or receiving end of a wireless communication system. Herein, the MIMO technology may be referred to as multi-antenna technology.
In a multi-antenna wireless communication system according to the related art, a rank indicator (RI), a precoding matrix index (PMI), channel quality information (CQI), etc. are defined as information fed back from a receiving end (for example, user equipment) to a transmitting end (for example, base station). Such feedback information may be referred to as channel status information (CSI). The user equipment may feed its preferred PMI and RI back to the base station on the basis of the result of a channel measured by the user equipment. In this case, the PMI and RI preferred by the user equipment, if used by the base station at a given channel status, correspond to downlink transmission rank value and precoder information, which may have the highest transmission rate. Also, CQI is calculated on the basis of the PMI/RI reported by the user equipment, and corresponds to a modulation and coding scheme (MCS) level applied to downlink transmission. In this case, the CQI may represent MCS level that provides a packet error rate within the allowable range if precoder information and rank value based on the PMI/RI reported by the user equipment are used.